Many questions are being asked to clarify how many modules can be fitted on Yeastar ( old Yealink ) PBXes. See the calculations below. S20 can directly have 2 Modules ( each module can have either 2 FXO or 2FXS , or 1FXO and 1FXS, or 1GSM ) – see the module pictures below. S20 can directly have 2 Modules – so 4 FXO S20 can directly have 4 Modules – so 8 FXO Neither S100 or S300 comes with FXO slots. We can add EX08 module which can…